1. Field of the Invention
The present invention relates to an apparatus and method, for use in a satellite-based communications network, for positioning single-offset zones in a spot beam coverage area to maximize call duration. More particularly, the present invention relates to an apparatus and method for maximizing call duration in a satellite-based communications network by segregating a spot beam coverage area into at least one single-offset zone, positioning the center of each respective single-offset zone to overlap a respective high user density area in the spot beam coverage area, and assigning to each single-offset zone a burst offset value based on propagation delay at its respective high user density area.
2. Description of the Related Art
A satellite communications network, such as a geosynchronous earth orbit mobile communications network, comprises at least one geosynchronous earth orbit satellite, a ground-based advanced operations center (AOC) and spacecraft operations center (SOC) associated with the satellite, at least one ground-based gateway station (GS), and at least one access terminal (AT), which is typically a hand-held or vehicle mounted mobile telephone. The satellite enables the access terminal to communicate with other access terminals, or with other telephones in the terrestrial public switched telephone network (PSTN), via the gateway stations under the control of the gateway stations. The AOC provides system-wide resource management and control functions for its respective satellite, and the SOC controls on-orbit satellite operations for its respective satellite.
To communicate with access terminals, the network controls a satellite to generate at least one spot beam, which is typically an L-band frequency signal, toward the surface of the earth. Each spot beam covers a predetermined geographic region of the earth, thus enabling access terminals within that region to communicate with the network via communications signals transmitted to the satellite over a carrier selected from a plurality of carriers assigned to the spot beam.
For example, when an access terminal places a call to another access terminal or to a terrestrial telephone, the access terminal generates and transmits a channel request message on a random access channel (RACH) at a frequency assigned to the spot beam to the satellite. Typically, a channel request message includes data representing a random number, which is used as an identifier for the access terminal sending the channel request message, as well as contention resolution and timing synchronization information.
The satellite includes a receiver which, under the control of the network, establishes time frames of a particular duration during which channel request messages are received. When a channel request message is received by the satellite during a particular time frame, and is thus received by the network, the network transmits data via the satellite to the access terminal to establish a communication link between the access terminal and network. The data includes access channel information (access grant channel information) indicating the frequencies of the carriers over which communication between the network and the access terminal is to occur during the call. Typically, a carrier of a particular frequency is assigned to service transmission of communications from the satellite to the access terminal, and a carrier of another frequency is assigned to service transmission of communications from the access terminal to the satellite. Hence, a pair of carriers service communication between the satellite and an access terminal.
Communication between the network and access terminal occurs in the form of signal bursts of a predetermined duration which are transmitted over the carrier pair designated by the access grant channel information between the access terminal and satellite. Signal bursts transmitted from the satellite to the access terminal, along with signal bursts being transmitted to other access terminals, if any, also assigned to a carrier in the carrier pair, are transmitted over the carrier in a time-division multiple access (TDMA) manner. That is, each signal burst being transmitted from the satellite to the access terminal is time-multiplexed with the signal bursts being transmitted by the satellite to the other access terminals in a TDMA frame of a particular duration, and transmitted over the carrier.
A TDMA frame includes a plurality of timeslots, which become occupied by the time-multiplexed signal bursts being transmitted. For example, a TDMA frame can include 24 timeslots, and each signal burst can be 3 timeslots long. Accordingly, a 24 timeslot TDMA frame can contain up to eight signal bursts which are being transmitted to eight respective access terminals (i.e., 8 signal bursts of 3 timeslots each), with each burst occupying three specific sequential timeslots of the TDMA frame. Naturally, a 24 timeslot TDMA frame can accommodate only four signal bursts which are each 6 timeslots in length, with each signal burst occupying six specific sequential timeslots of the TDMA frame.
Upon receiving its appropriate signal burst transmitted from the satellite, each access terminal transmits a signal burst back to the satellite in a TDMA frame over the other carrier in the carrier pair. An access terminal begins transmitting its respective signal burst at an appropriate instant in time after the instant in time at which the access terminal began receiving its respective signal burst transmitted from the satellite as described above. Because the transmitter/receiver of an access terminal is a typically a half-diplexer which permits only signal transmission or signal reception at any given time, the time period in which the access terminal transmits a signal burst can not overlap the time period in which the access terminal is receiving a signal burst.
For example, in a typical TDMA based GEO mobile satellite system discussed above, each access terminal must separate its burst transmission and receive times by a certain time period, known as minimum guard time Tgt min, so that the half-diplexer can receive and transmit bursts properly. By insuring that sufficient guard time is provided, the xe2x80x9cvalid call durationxe2x80x9d period for the access terminal, which is the time duration from call setup until blockage occurs, can be maximized.
An access terminal can attempt to comply with the minimum guard time requirement, and thus attempt to maximize valid call duration, by setting the timing separation between transmission and receive bursts to be longer than Tgt min at the beginning of each call. However, during a call, it is common for the satellite and the access terminal to move relative to each other due to satellite diurnal motion and access terminal mobility. Due to this relative movement, the timing of the downlink and uplink bursts at the access terminal move relative to each other during a call, so that the access terminal eventually suffers from a shortage of Rx/Tx guard time.
To avoid call blockage, the system can perform a timeslot handover operation as known in the art, in which the blocked call is handed over from one timeslot to another. Although timeslot handover can be effective in minimizing call blocking, the hardware and software needed to perform the handover operations greatly complicates the system design and also requires additional signaling.
Another way to increase valid call duration is to properly select the satellite burst offset on per call basis, so that at any particular access terminal, the uplink burst has a maximum distance to both bursts received in the downlink (one earlier, one later), thus maximizing call duration. However, from traffic resource management point of view, it is preferable that a single satellite burst offset be shared by all access terminals in the coverage area supported by this single offset, which could be the entire spot beam coverage area or one offset zone from among a plurality of offset zones in a spot beam coverage area. As a result, this offset may result in a longer guard time for some access terminals on one side of the single offset zone, but a shorter guard time for some other access terminals on the opposite side of the offset zone.
Accordingly, a need exists for a system capable of increasing valid call duration without performing timeslot handover operations.
An object of the present invention is to provide an apparatus and method, for use in a satellite-based communications network, for increasing valid call duration in the network without performing timeslot handover operations.
Another object of the present invention is to provide an apparatus and method, for use in a satellite-based communications network, for positioning the offset zones in spot beam coverage area such that the center of each offset zone overlaps a respective area within the spot beam coverage area having high user density, to maximize call duration.
A further object of the present invention is to provide an apparatus and method for maximizing call duration in a satellite-based communications network by segregating a spot beam coverage area into at least one single-offset zone, positioning the center of each respective single-offset zone to overlap a respective high user density area in the spot beam coverage area, and assigning to each single-offset zone a burst offset value based on propagation delay at its respective high user density area.
These and other object of the present invention are substantially achieved by providing an apparatus and method for increasing call duration time in a satellite-based communications network, employing a spot beam segregator and an offset zone arranger. The spot beam segregator segregates a coverage area of a spot beam generated by a satellite in the network into at least one offset zone based on respective propagation time periods required for signals to travel between the satellite and respective different locations in the coverage area. The offset zone arranger arranges each offset zone such that each respective center of each one offset zone overlaps a respective geographical location in the coverage area, with each respective geographic location having a respective access terminal density greater than any other geographical region within its respective offset zone. Each offset zone is thus assigned a burst offset value based on propagation delay at its respective high user density area to maximize call duration.